Current technological developments are very much directed at new display screens that are flexible and light-weight and are capable of covering large areas. Such display devices have the potential of offering high brightness for lighting applications, as well as bleach-resistant colours and long device lifetimes. In particular, such display devices offer the advantage of cheap production costs and low energy consumption during their lifetime, due to low operating voltages. Production costs can be low because many of the components required to make such devices are based on cheap organic compounds and polymers. OLEDs are therefore highly attractive candidates to fulfil the requirements of the next advance in display technology.
The function of OLEDS has been described numerous times [see for example: J. Shinar (ed.): Highly Efficient OLEDs with Phosphorescent Materials. Wiley-VCH, Weinheim 2008; Z. H. Kafafi: Organic Electroluminescence. Taylor & Francis, Boca Raton 2005; X. H. Yang, D. C. Müller, D. Neher, K. Meerholz, Adv. Mater. 2006, 18, 948; H. Yersin, Top. Curr. Chem. 2004, 241, 1], for example also in U.S. Pat. Nos. 4,539,507, 5,151,629 and WO 98/27136. A device showing green electroluminescence with good efficiency based on a simple coordination compound, tris(8-hydroxyquinolinato)aluminium, was reported in 1987 [C. W. Tang, S. A. van Slyke, Appl. Phys. Lett. 1987, 51, 913 and U.S. Pat. No. 5,151,629], and it was shown that a triple-layer structure improved the device efficiency. The content of these publications is incorporated herein by reference.
As is known, OLEDs generally comprise, in sequence, an anode, optionally a hole-transporting zone, an emissive zone capable of emitting light, and a cathode. The arrangement may suitably be supported on a substrate. An electron-transporting zone may be present, between the emissive zone and the cathode. OLEDs are typically multilayer structures with each component part forming a layer or part of a layer. Depending on which side or sides of the device is/are to emit the light, layers may be independently selected to be transparent, translucent or opaque.
An embodiment of an OLED is shown schematically in FIG. 1 of the accompanying drawings, to illustrate a typical sequence of layers. In this embodiment, a glass substrate is covered by a thin, optically transparent, layer of indium tin oxide (ITO), which acts as anode. A metal or metal alloy of low work function acts as cathode. The cathode and anode are separated by several layers of different organic molecules which are able to conduct charges, and provide the hole-transporting and emissive zones. Holes are injected into the organic layers from the anode, and electrons are injected from the cathode. The holes and the electrons migrate in opposite directions in the layers of the organic molecules and bind to form excitons. In the illustrated embodiment the anode injects into a hole-transporting layer (hole-transporting zone), which may, for example, comprise a hole-transporting material such as PEDOT:PSS [poly(3,4-ethylenedioxythiophene)/polystyrene sulfonate polymer mixture], while the cathode injects into an electron-injection layer, which may, for example, comprise an electron-transporting material such as a metal oxide, salt or organic electron-transporting compound. Both the hole-transporting layer and the electron-injection layer serve to block the escape of the opposite-sign charge carrier. Between these layers is the emissive layer, which is a layer of organic compounds in which excitons form in a mixture of spin-0 singlet and spin-1 triplet states.
Layer devices such as that illustrated in FIG. 1 are built up by successive deposition of the layers on the substrate. The deposition technique for each layer is selected from a range of available techniques. For example, the cathode is typically generated by evaporating metal vapour on the top surface of the previously deposited layers.
A common approach to forming the material of an emissive layer is to embed emitter molecules in a wider-bandgap organic matrix, usually at a level of 1-10 weight-percent. The role of the matrix is to allow excitons and charges to migrate to the emitter molecules. In use of the device, through charge or energy transfer, these emitter molecules are promoted to an excited state, which relaxes with the emission of light. This light can be of varying colour and can include white light.
The emitter molecules can be of various types, such as, for example, organic compounds with no metal atoms, organometallic complexes of heavy transition metals, and metal coordination complexes. The key role of the heavy metals in the complexes is to enhance spin-orbit coupling and thus allow luminescence from the normally dark triplet exciton state, significantly increasing the achievable photon quantum yield. The device efficiency is improved when the emitters have excited states with short life times, reducing competition with non-radiative decay channels.
A wide range of such metal complexes for applications as photo-emitters and based on polydentate ligand frameworks is described in WO2004/081017, the content of which is incorporated herein by reference. Particularly widely used as emitters are complexes of expensive noble metals, especially those of iridium, platinum and gold in high oxidation states, which are used in combination with polydentate chelating aromatic and heteroaromatic ligands. Emitter compounds based on these metals have been extensively patented and have for example been described in WO 2006/070896, U.S. Pat. 2009/0278453, WO2014/023377, WO2013/097920, WO2014/094960, WO2014/094961, WO2014/094962 and U.S. Pat. 2011/0012093, the contents of which are incorporated herein by reference.
Many copper complexes are known to become luminescent when excited with UV light. These complexes contain copper in the coordination number three or, most commonly, coordination number four in a distorted tetrahedral geometry and are of the form Cu2X2L4, where L stands for a monodentate or part of a polydentate phosphine ligand. Copper and silver complexes of this type are for example described in WO 2014/102079, and are based on binuclear halide-bridged structures and four-coordinate metal centres. Similar copper and silver complexes of bidentate phosphines coordinated to bidentate nitrogen donor ligands in place of halide ligands are described in WO 2014/108430. The photophysical characteristics of copper complexes have recently been described in more detail in a number of reviews [N. Armaroli, G. Accorsi, F. Cardinali, A. Listorti: “Photochemistry and photophysics of coordination compounds: Copper”, Topics in Current Chemistry 2007, 280, 69-115; M. Wallesch, D. Volz, D. M. Zink, U. Schepers, M. Nieger, T. Baumann, S. Braese: “Bright coppertunities: multinuclear complexes with N—P ligands and their applications”, Chemistry—a European Journal 2014, 20, 6578-6590; the contents of these publications are incorporated herein by reference].
There is a well-recognised need for emitters based on cheap, earth-abundant metals. Materials based on earth-abundant metals offer, on the one hand, the advantage of reduced costs, and on the other hand, they mitigate against possible supply limitations that are inherent in the use of rare noble metals.
The purpose of using heavy-metal emitters such as iridium is to enable fast inter-system crossing, so that the ligand-based excited triplet states can be harvested in the form of emitted light. However, excited triplet states in close proximity are known to suffer from triple-triplet annihilation, reducing electroluminescence yield. In addition, many existing organometallic emitter materials suffer from strong concentration quenching. Dilution within the host matrix is therefore necessary to achieve efficient luminescence, and gradual migration and aggregation of emitter molecules leads to device failure. There is therefore a need to develop emitters with high quantum efficiency in the solid state, reducing the impact of aggregation.
There is a well-recognised need for emitters having high external quantum efficiency, preferably in excess of 15%, and devices incorporating them, e.g. OLEDs.
There is further a need to develop complexes that are readily soluble in common organic solvents in order to enable these complexes to be incorporated into the electronic device by cheap solution processing methods. Solution processing means in this context that the compound is capable of being dissolved, dispersed or suspended in a liquid medium. Such a solution, dispersion or suspension should be suitable for producing layer structures in OLED devices by coating or printing from a liquid phase, such as, for example, spin-coating, ink-jet printing or suitable alternative techniques.
Furthermore, there is a need for emitter materials that are simple and cheap to synthesize, keep their intended composition in solution and/or do not undergo ligand rearrangement reactions during processing.
Production costs of electroluminescent devices can be reduced using solution-based processing techniques, including a combination of solution processing and vacuum deposition techniques. With lowered production costs, the use of noble metal compounds as emitter materials can become economically attractive. Production of electroluminescent devices incorporating, as emitter materials, compounds of gold in the oxidation state +III and bonded to polydentate chelating aromatic ligands has been described, for example, in Chemistry A European Journal 2014, vol. 20, p. 15233-15241, in Advanced Materials 2014, vol. 26, p. 2540-2546, and in the Journal of the American Chemical Society 2014, vol. 136, p. 17861-17868, the contents of which are incorporated herein by reference. However, the external quantum efficiencies of these devices were below 10-15%.
Attempts to fabricate electroluminescent devices based on complexes of gold in the oxidation state +I by solution processing methods have also been reported, but were found to suffer from low quantum yields. This has been shown in devices incorporating binuclear gold(I) complexes of chelating phosphine ligands, reported in Chem. Commun., 2000, 53-54, the contents of which are incorporated herein by reference, which gave quantum yields of only 0.1-0.2%. The use of tetranuclear gold(I) triphosphine complexes, was reported in Inorganic Chemistry 2014, vol. 53, p. 12720-12731, the contents of which are incorporated herein by reference, and the use of nanocrystals of metal-metal bonded gold salts of the composition {[Au(carbene)2][Au(CN)2]}n, as emitter materials, was described in Chemical Science 2014, vol. 5, p. 1348-1353, the contents of which are incorporated herein by reference. The external quantum efficiencies of these devices did not exceed 4%.
There is therefore a need for emitter materials that do not suffer from concentration quenching, and which are capable of giving solution-processable electroluminescent devices with maximum external quantum efficiencies of 15% or higher.
Surprisingly, it has now been found that certain types of carbene complexes of copper, silver and gold in the oxidation state +I as described in further detail below are strongly photoemissive, which meet the performance requirements for emitter materials as outlined above and/or have solid-state quantum yields of 80% or higher. Unlike previously reported photoemissive copper complexes, these compounds have a linear, two-coordinate geometry. The complexes show excellent thermal stability and are soluble in all commonly used organic solvents. These characteristics render them suitable for use in solution processing and liquid deposition techniques for forming layered structures, for example OLEDs. Depending on their composition, these complexes may also be volatile and therefore may be suitable for gas or vapour phase processing in the production of devices such as OLEDs.
In Chem. Commun., 2016, 52, 6379-6382, published on 12 Apr. 2016, the contents of which are incorporated herein by reference, certain photophysical properties of a group of linear two-coordinate copper and gold cyclic alkyl amino carbene (CAAC) halide complexes, defined as AdL-M-X and individually designated 1a, 1b, 1c, 2a, 2b and 2c according to the following scheme:
are described (see, for example, Tables 1 and 2, FIGS. 1 to 3).
It is reported in the said publication that the copper compounds show photoluminescence with solid-state quantum yields of up to 96%, the light emission being independent of temperature over the range T=4-300 K. The photoluminescence is reported to occur very efficiently by prompt rather than delayed fluorescence, with lifetimes in the sub-nanosecond range. The solid-state photoluminescence quantum yield of the copper compounds is: compound 1a 0.96 (96%); compound 1b 0.61 (61%); compound 1c 0.28 (28%). The solid-state photoluminescence quantum yield of the gold compounds is: compound 2a 0.09 (9%); compound 2b 0.13 (13%); compound 2c 0.18 (18%).
The said publication Chem. Commun., 2016, 52, 6379-6382 is a disclosure made less than six months before the filing date and second priority date of the present patent application and the invention claimed herein, by at least one of the inventors of the present invention and/or by at least one other who obtained the disclosed subject-matter directly or indirectly from at least one of the inventors of the present invention. The invention embodied in the said publication is therefore being claimed in the present patent application for the purposes of patent or like protection in territories where such prior publications are excluded from the prior art for the analysis of novelty and inventive step.